The present invention relates to the drying of moisture-laden agricultural commodities, and in particular, to a recycle control for recirculating grain dryers.
Recirculating grain dryers, such as the device disclosed in the Botkins U.S. Pat. No. 4,048,727, are well known in the field of agriculture, and include at least one porous grain column constructed of a material such as a wire cloth screen which retains the grain therein, and permits hot air to be circulated therethrough. An arrangement is provided which recirculates at least a portion of the hot air which is blown into the grain and is otherwise exhausted into the atmosphere. Such recirculating grain dryers include a recirculating blower, which, in the recirculating mode, draws in preheated air from a cooling section of the dryer after passing through the grain, and/or a lower portion of a heating or drying section of the dryer. The preheated air is further heated, by a burner and is then blown back into the drying section. Because the drying air is partially preheated during the recirculating mode, less energy is required to ring the drying air up to the required temperature, thereby realizing a substantial savings in heating energy.
Recirculating grain dryers must be carefully regulated to control the temperature and humidity of the drying air. Excessively high drying air temperatures increase the hazard of fire or explosion, particularly when the commodity being dried has a great deal of free chaff or dust. Excessively high humidity in the drying air reduces efficiency of drying and may completely stop the drying process in addition to potentially doing damage to the components of the dryer. Further, the controls must be readily adjustable to accommodate for different atmospheric conditions, and various degrees of moisture content in the harvested commodity.
It is desirable that the degree or extent of recirculation be variable in accordance with the type of commodity to be dried. All crops contain some percentage of chaff which during drying becomes suspended in the recirculated air and is deposited on the porous, wire cloth grain columns. When the chaff buildup on the grain columns becomes excessive, the pores therein are blocked, and the entire dryer must be shut down and cleaned. Some commodities, such as milo, contain such a high chaff content that it is not economically feasible to operate the dryer with any degree of recirculation due to the rapid buildup of debris on the grain columns. Other commodities have a very low chaff content, such that they can typically be dried with the economical, full-recirculation mode. Hence, it is preferable that recirculating grain dryers include a recycle control which can vary the degree of recirculation from full to no recirculation, and provide for any possible partial recirculation therebetween.